A theory created by W.D.Hamilton and populised by Richard Dawkins in his book The Selfish Gene which postulates that the basic unit of selection in nature is not the species, or organism, but the individual genes in an organism, and that any action in nature, however altruistic it may be, is in fact done to spread the genes that caused it.

This is usually stated in contrast to whole organism and species selection, both of which leave unexplained phenomena. The selfish gene theory can explain such problems as altruism and social insects, something the other theories find very hard to do.

One interesting mathematical idea drawn by W. D. Hamilton and Richard Dawkins based on the Selfish Gene model is the idea that people are altruistic in proportion to how closely they are related to someone. So I am 100% altruistic to myself (duh), and in comparison I am 100% altruistic to an identical twin, 50% altruistics to a parent or sibling or offspring, 25% altruistics to an uncle or aunt or nephew or nice, 12.5% altruistic to a cousin, etc. I think it was the biologist J. B. S. Haldane who said, "Would I die for my brother? No, but i would for two brothers. Or eight cousins."